Avoid user heap to override the minimum stack size

Minimum stack size is defined in linker script:
_Min_Stack_Size = 0x400;; /* required amount of stack */

If more stack is requested, then user have to ensure that heap
and stack can fit in the SRAM.

Fix #307

Signed-off-by: Frederic.Pillon <frederic.pillon@st.com>

Author: fpistm

cores/arduino/syscalls.c

@@ -29,6 +29,8 @@ register char * stack_ptr asm("sp");
 
 __attribute__((weak))
 caddr_t _sbrk( int incr ) {
+  extern char _estack; /* Defined in the linker script */
+  extern char _Min_Stack_Size; /* Defined in the linker script */
   extern char _end; /* Defined by the linker */
   static char *heap_end = &_end ;
   char *prev_heap_end = heap_end;
@@ -38,6 +40,11 @@ caddr_t _sbrk( int incr ) {
     errno = ENOMEM;
     return (caddr_t) -1;
   }
+  /* Ensure to keep minimun stack size defined in the linker script */
+  if (heap_end + incr >= (char*)(&_estack - &_Min_Stack_Size)) {
+    errno = ENOMEM;
+    return (caddr_t) -1;
+  }
 
   heap_end += incr ;
   return (caddr_t) prev_heap_end ;